ABSTRACT Dysregulated ROR-?t-mediated IL-17 expression is strongly associated with chronic inflammation. However, the molecular mechanisms by which ROR-?t functions and how its stability is regulated remain elusive. Our preliminary studies suggest that under steady-state conditions in the gut, ROR-?t is sumoylated in Th17 cells by UBC9 (SUMO-E2 ligase) and TRIM55 (SUMO-E3 ligase). Additionally, we found that under steady-state conditions, a novel NCOA6-HDAC2 repressor complex binds to ROR-?t. Upon T cell receptor stimulation, de- sumoylation removes this repressor complex, resulting in IL-17 transcription. To prevent a prolonged Th17 response following stimulation, ROR-?t is phosphorylated by the kinase Pak2, which promotes Itch-mediated ROR-?t ubiquitination and subsequent degradation. Based on these findings we hypothesize that that ubiquitination and sumoylation of ROR-?t are critical molecular events that regulate Th17 responses and that can be targeted therapeutically. In AIM 1, we will determine the mechanism by which sumoylation of ROR-?t represses IL-17 transcription. By using newly generated CD4 T cell-specific TRIM55-/- mice, we will investigate the mechanism by which the NCOA6/HDAC2 complex utilizes histone deacetylation to repress IL-17 expression. In AIM 2, we will determine the mechanism by which phosphorylation promotes Itch-mediated ROR-?t ubiquitination. Using Pak2-/- mice, we will investigate how the phosphorylation-dependent conformational change of ROR-?t promotes its degradation. In AIM 3, we will target the Itch-ROR-?t-IL-17 pathway to inhibit excessive inflammation. We will test the therapeutic potential of a small-molecule Itch activator to inhibit colonic inflammation. Completion of these studies will lead to a clear understanding of the molecular mechanisms by which ROR-?t function and its stability are regulated to prevent chronic inflammation. This knowledge should lead to improved therapeutic strategies to target the ROR-?t-IL-17 pathway in human inflammatory diseases.